(1) Background of Invention
The present invention was made as a product of research and development efforts conducted by ALOKA CO., LTD., a Japanese corporation, related to the “direction finding gamma ray detector” project commissioned under the auspices of the Japan Science and Technology Agency, an agency of the Government of Japan.
(2) Related Art
Monitoring posts are commonly provided as radiation measurement apparatuses, for example, along the periphery of facilities such as nuclear power plants, a nuclear fuel processing facilities, etc., or at measurement points determined by local governments, and continuously execute radiation measurement at the location at which they are installed. With this system, the presence or absence of abnormalities is monitored at all times. At such a monitoring post, it is usually desirable that the direction of emission of any detected radiation be automatically identified. As a method for identifying the incident direction of radiation, a method is known in which a directional detector is rotated. In a typical directional detector, because a collimator and a shielding member comprising lead or the like are provided, a large and relatively complicated mechanism is required for rotating the detector. It is also desirable to identify the incident direction of radiation measurement apparatuses other than monitoring posts (such as, for example, a floor-mounted survey monitor). Reference 1 (JP 2004-191327 A), Reference 2 (JP 2004-361290 A), and Reference 3 (Yoshiyuki SHIRAKAWA, “Development of nondirectional detectors with NaI(Tl)”, RADIOISOTOPES, Vol. 53, No. 8, pp. 445-450, August 2004) all disclose that a peak is identified in each of a plurality of spectra obtained using a plurality of types of scintillators, a ratio of counts for the plurality of peaks is determined, and the incident direction (incoming direction) of the radiation is identified based on the ratio. Japanese Patent Application No. 2005-014578 is related to the present application.
However, when the method of identifying a peak in a spectrum as noted above is used, there is a problem that the reliability of the calculation is reduced if the peak is unclear or there is a problem with the precision of the peak identification. Although it is possible to wait and not execute the calculation until after a peak has clearly appeared, such a configuration prevents quick processing. It is desired that it be possible to discriminate among radiation from a plurality of sources simultaneously incident upon the detector. Moreover, it is desired that it be possible to identify the energy, level, and/or energy interval of radiation in addition to identifying its incident direction.